"We had previously shown that ultraviolet light has an effect on mood that tanners value," study lead author Dr. Mandeep Kaur said in a prepared statement. "Now, in this small study, we've shown that some tanners actually experience withdrawal symptoms when the 'feel-good' chemicals are blocked."

The study included eight frequent tanners (eight to 15 times a month) and eight infrequent tanners (no more than 12 times a year). The participants were given either a placebo or a drug (naltrexone) that blocks the effects of endorphins and other opioids. They then used both UV and non-UV tanning beds.

At higher doses of naltrextone (15 milligrams), frequent tanners showed a preference for UV tanning and four of the eight frequent tanners reported nausea or jitteriness. None of the infrequent tanners who took the drug reported these symptoms.

"The finding was unexpected and is consistent with the hypothesis that frequent tanning may be driven in part by a mild dependence on opioids, most likely endorphins. The nausea and jitteriness induced by the medication are consistent with symptoms of mild opiate withdrawal," senior researcher Dr. Steven Feldman, a professor of dermatology, said in a prepared statement.

The study appears in the April issue of the Journal of the American Academy of Dermatology.